In del_vpngateway_route, delete all vpngateway routes
Depending on the default network settings, vpnc-script can add several vpngateway routes (e.g. via different interfaces), but when disconnecting, it deletes only one of them.
Let us delete all of them to restore the default settings: do successive deletes, stopping as soon as the command fails (which occurs when there are no remaining vpngateway routes).
Fixes #65.